When E.K. Hornbeck says, " I may be rancid butter, but i'm on your side of the bread," he means
that his view on the matter of evolution is not the "accepted" view of the people of Hillsboro, however he does share the same view as Rachel and Cates. When people hear of rancid butter, they think of something nasty, and it sickens them. This is almost how people of Hillsboro feel when they hear the biological reasoning to evolution. The bread also symbolizes the people of Hillsboro because of the fact that there are two sides on a piece of bread. In Hillsboro, there are two sides to the argument on evolution theories. Hornbeck is on the same side of the bread as Cates and Rachel because they all share the same beliefs.